It is known to use one-piece pistons for internal combustion engines. Typically, one-piece pistons have a crown connected to a skirt.
It is known to forge the crown and skirt as one-piece. The forged one-piece piston is then machined to form the component parts of the crown such as piston ring grooves and an annular wall having a recess about its periphery. The annular wall is an outer portion which extends downwardly from the bottom of a piston ring zone to an apex of a piston pin bore. The remaining outer portion material defines a skirt.
Forging the crown and skirt as one-piece undesirably requires excess material during the forming process that must later be machined away. To create the annular recess, for example, an area approximately the height of an upper crown must be removed. Machining away the excess to form the component parts of the one-piece piston, therefore, can be both cumbersome and expensive.
The weight of the piston depends on the weight of the single piece of forged material. Using less material, however, may ultimately decrease the utility and strength of the piston. Using commercially available lighter material such as aluminum tends to impose design constraints, such as the depth of the combustion bowls and the location of the ring grooves.
Additionally, the piston crown endures at least two separate loads in service. The upper crown endures both thermal and mechanical loads created by the combustion gas gathered in the combustion bowl at the top of the upper crown. The lower crown, however, endures a side load from the motion of the crankshaft. In the forged piston, the strength needed to endure the two different loads is determined by the strength of the single material chosen. While steel, for example, is strong enough to handle both loads, its weight and expense makes it inefficient for many applications.